Okay, let's break down NEC 2017 Article 240.21(C) and how it applies to your situation, specifically focusing on the placement of the secondary fused disconnect for your transformer.

Understanding NEC 240.21(C) - Secondary Conductors

Article 240.21(C) covers the overcurrent protection requirements for transformer secondary conductors. It allows for the secondary conductors to be protected at a location other than where they originate (i.e., the transformer secondary terminals), but there are conditions that must be met. The specific subsections of 240.21(C) are crucial: 240.21(C)(1) - Secondary Conductors Not Over 10 ft. Long

This is likely the section you're applying, given the "10 feet" you mentioned. This section states that the overcurrent protection is not required at the point where the secondary conductors originate, provided the length of the conductors does not exceed 10 ft and meets all the following conditions:

(1) Ampacity

The ampacity of the secondary conductors shall not be less than: (a) The calculated load in accordance with Article 220, and (b) The rating of the overcurrent device at the termination of the secondary conductors. This means your conductors must be sized large enough to handle the load and be protected by the downstream breaker/fuse.

(2) Terminated by an Overcurrent Device

The secondary conductors are terminated in a single overcurrent device that limits the load to the ampacity of the conductors.

(3) Enclosed in Raceway or Enclosure

The secondary conductors are enclosed in a raceway or enclosure from the transformer secondary to the overcurrent device.

240.21(C)(2) - Secondary Conductors Not Over 25 ft. Long

This allows for a longer run, but with stricter requirements, including a higher ampacity requirement (not less than 1/3 of the primary conductor overcurrent device). This is less likely to be applicable to your case, but you should be aware of it.

Applying to Your SituationYou have a 15 kVA, 480V to 208/120V transformer. You're using the 10-foot tap rule [240.21(C)(1)]. Let's analyze the implications for your disconnect placement:1.

Maximum Length

Yes, the key point is that you are using the secondary conductors not over 10 ft.2.

Disconnect Placement

The NEC does not explicitly prohibit the fused disconnect from also being located in the ceiling. However, you must ensure it is accessible. This is a critical point. Here's why and what to consider:

Accessibility (NEC 110.26)

Electrical equipment, including disconnects and overcurrent devices, must be readily accessible. "Readily accessible" is defined in Article 100 as "Capable of being reached quickly for operation, renewal, or inspections without requiring those to whom ready access is requisite to climb over or remove obstacles or to resort to portable ladders, and so forth."

Practical Considerations

Putting the disconnect in the ceiling might make it not readily accessible, especially if it requires a ladder every time it needs to be accessed. While a ceiling can be "accessible," the use of a ladder to reach it could violate the intent of the definition. Regular maintenance and operation may require this.

Therefore, if you can meet the readily accessible requirements (e.g., a platform, a permanent ladder, or a low enough ceiling where a short, readily available step stool suffices), the disconnect can technically be in the ceiling. However, from a design and practical maintenance standpoint, it's often better to locate the disconnect in a readily accessible location at a lower level.Key Considerations and Recommendations

Accessibility is Paramount

This is the most critical factor. Think about who will need to operate and maintain this equipment. Can they do so safely and easily?

Working Clearance (110.26)

If the disconnect is in the ceiling, ensure there's sufficient working space (height, width, and depth) around it to comply with NEC 110.26. This includes adequate headroom.

Local Amendments

Your local jurisdiction may have amendments to the NEC that are stricter than the national code. Always check with your local electrical inspector for any specific requirements in your area.

Conductor Protection

Be certain the conductors are properly sized and protected from physical damage within the raceway.

In summary, while technically the disconnect could be in the ceiling, provided it is considered readily accessible, it's often better to install it in a more easily accessible location for safety, maintainability, and ease of operation. Prioritize safety and practicality in your design.* Also, make sure to verify the above information with your local electrical inspector before proceeding. They have the final authority on code interpretation in your area. Flag for review